Nonaqueous electrolyte cells using lithium as a negative electrode active material and a nonaqueous solvent such as an organic solvent as an electrolyte have advantages in that self-discharge is low, a nominal potential is high and storability is very excellent.
Typical examples of such nonaqueous electrolyte cells include lithium manganese dioxide primary cells and they are widely used as current sources for clocks and memory backup of electronic instruments because of their long-term reliability.
However, conventionally used nonaqueous electrolyte cells are primary cells which can be used only once. On the other hand, with recent wide spread of video cameras and small-sized audio instruments, there has been an increased need of secondary cells which can be use for long, conveniently and economically by repeated use. For this reason the research development of nonaqueous electrolyte secondary cells has been continued.
Of nonaqueous electrolyte secondary cells, a nonaqueous secondary cell which is provided with (a) a negative electrode consisting essentially of a carbonaceous material as a carrier for a negative electrode active material , said carrier being capable of being doped and dedoped with lithium and (b) a positive electrode comprising lithium manganese complex oxide as an essential positive electrode active material, has been proposed by one of the present inventors with other inventors in Japanese Patent Application No. Hei. 1-220216. This cell has a highly expected applicability because dendrite precipitation of lithium does not occur on the surface of the negative electrode, the pulverization of lithium is inhibited, the discharge characteristics are excellent and the energy density is high.
However, a nonaqueous electrolyte secondary cell such as described above has disadvantages in that the cell capacity is prove to decrease because lithium doped into the carbonaceous material used as a negative electrode active material cannot be efficiently dedoped upon discharge.
The cause of the decrease in cell capacity is explained below by reference to FIG. 6.
FIG. 6 shows one example of a charge-discharge curve of a nonaqueous electrolyte secondary cell such as described above and illustrates the changes of the charge and discharge potentials of the negative and positive electrodes and the changes of the cell charge and discharge voltages caused by charge and discharge, respectively.
The charge and discharge of the cell in FIG. 6 are effected at the same constant current density.
In the above-described nonaqueous electrolyte secondary cell it is necessary to charge the cell just after the assembling of the cell because the carbonaceous material as a carrier for the negative electrode active material has not been doped with lithium.
During a charge, lithium is dedoped from the positive electrode active material, lithium-manganese oxide LiMn.sub.2 O.sub.4 and the carbonaceous material as the carrier for the negative electrode active material is doped with the dedoped lithium.
Such reactions taking place at the positive electrode and negative electrode during a charge are shown by the following equations (1) and (2):
Positive electrode: EQU Li.sub.1.0 Mn.sub.2 O.sub.4 .fwdarw.Li.sub.1-y Mn.sub.2 O.sub.4 +y.multidot.Li.sup.+ +y.multidot.e.sup.- ( 1)
Negative electrode: EQU C+y.multidot.Li.sup.+ +y.multidot.e.sup.- .fwdarw.Li.sub.y C(2)
The change in the positive electrode charge potential during the reaction of equation (1) above is shown by the broken line in FIG. 6 and the change in the negative electrode charge potential is shown by the alternate dot and dash line in FIG. 6.
During discharge, lithium is dedoped from the negative electrode and the positive electrode is doped with the dedoped lithium.
Such reactions during discharge are shown by the following equations (3) and (4):
Negative electrode: EQU Li.sub.y C.fwdarw.Li.sub.y-z C+z.multidot.Li.sup.+ +z.multidot.e.sup.-( 3)
Positive electrode: EQU Li.sub.1-y Mn.sub.2 O.sub.4 +z.multidot.e.sup.- .fwdarw.Li.sub.1-y+z Mn.sub.2 O.sub.4 ( 4)
The change in the negative electrode discharge potential during the reaction of equation (3) above is shown by the alternate dot and dash line in FIG. 6 and the change in the positive electrode discharge potential during the reactions of equation (4) is shown by the broken line in FIG. 6.
In equation (3) above showing the discharge reaction of the negative electrode the amount Z of lithium dedoped from the carbonaceous material used as the carrier for the negative electrode active material is smaller than the amount y of lithium with which the carbonaceous material is doped during charge in accordance with equation (2) above (z&lt;y). This can easily be understood by considering that in an embodiment shown in FIG. 6 the charge period of four hours corresponds to the amount y of lithium with which the carbonaceous material of the negative electrode is doped and the discharge period of about two hours forty minutes corresponds to the amount z of lithium dedoped from the carbonaceous material of the negative material.
Therefore, the above-described nonaqueous electrolyte secondary cell has a low charge-discharge efficiency (z/y.times.100) at the carbonaceous material of the negative electrode and the cell capacity , which is regulated by the negative electrode becomes small.